This invention relates to servo-motors of the vacuum-suspended type principally intended for use in the braking system of road vehicles and the like.
Previously known forms of servo-motor of the type concerned typically include a casing divided by a movable wall member into two power chambers, that is, into a first chamber to which vacuum is always communicated and a second chamber which is selectively placed into communication with the first chamber and the external atmosphere by control valve means, which is operable by an input member slidably fitted in the movable wall member. An output member is operatively connected with the movable wall member on the front side thereof and movable under the effect of the pressure difference occurring between the first and second power chambers when the second power chamber is placed in communication with the external atmosphere.
In such forms of servo-motor, in order to improve the responsivity to the input and obtain an input-output characteristic free from any substantial hysteresis, it is required, among other things, that any loss of vacuum in the first power chamber, when the second power chamber is changed in state of pressure from vacuum to atmospheric under the action of the control valve, is effectively prevented, that the change in state of the second power chamber is effected rapidly and that the resistance to the axial sliding movement of the input member is held to a minimum.